Free piston internal combustion engines include one or more pistons which are reciprocally disposed within corresponding combustion cylinders. However, the pistons are not interconnected with each other through the use of a crankshaft. Rather, each piston is typically rigidly connected with a plunger rod which is used to provide some type of work output. For example, the plunger rod may be used to provide electrical power output by inducing an electrical current, or fluid power output such as pneumatic or hydraulic power output. In a free piston engine with a hydraulic output, the plunger is used to pump hydraulic fluid which can be used for a particular application. Typically, the housing which defines the combustion cylinder also defines a hydraulic cylinder in which the plunger is disposed and an intermediate compression cylinder between the combustion cylinder and the hydraulic cylinder. The combustion cylinder has the largest inside diameter; the compression cylinder has an inside diameter which is smaller than the combustion cylinder; and the hydraulic cylinder has an inside diameter which is still yet smaller than the compression cylinder. A compression head which is attached to and carried by the plunger at a location between the piston head and plunger head has an outside diameter which is just slightly smaller than the inside diameter of the compression cylinder. A high pressure hydraulic accumulator which is fluidly connected with the hydraulic cylinder is pressurized through the reciprocating movement of the plunger during operation of the free piston engine. An additional hydraulic accumulator is selectively interconnected with the area in the compression cylinder to exert a relatively high axial pressure against the compression head and thereby move the piston head toward the top dead center (TDC) position.
In a free piston internal combustion engine as described above, the plunger rod is slidingly carried by a pair of bearings/seals which are respectively disposed between the combustion cylinder and the compression cylinder, and the compression cylinder and the hydraulic cylinder. Each bearing/seal allows reciprocating movement of the plunger rod while at the same time sealing around the plunger rod to fluidly separate the associated adjacent cylinders. Since the plunger rod is slidingly carried by the pair of bearings/seals, the longitudinal axis of the plunger rod defines the axis of reciprocating movement of the piston. Because of manufacturing tolerances, etc., it is possible that the piston head may not be disposed exactly concentrically with the longitudinal axis of the plunger rod. Alternatively, it is possible that the longitudinal axis of the combustion cylinder may not be disposed exactly concentric with the longitudinal axis of the plunger rod carried by the pair of bearings/seals.
The present invention is directed to overcoming one or more of the problems as set forth above.